The invention relates to a method for determining the fatigue strength of a corrodible connecting part disposed between at least two overlapping construction parts, which connecting part, during its intended use, being subjected to a certain corrosive environment as well as to a predetermined, repeated mechanical stress, with the use of diagrams (stress-number curves) that illustrate the magnitude of the repeated stress as a function of the number of load repetitions, until the connecting part is destroyed.
The construction of multi-part machines, vehicles and the like requires the determination of the operating durability of connecting parts that hold together the individual parts under the stresses occurring during the intended use of the parts in order to preclude a premature failure. The material and/or the dimensions of the connecting parts should therefore assure a service life of, for example, at least 30 years under normal operating conditions.
The fatigue strength of connecting parts can be ascertained with the use of stress-number curves. In this test, the connecting part is subjected to a specified, repeated mechanical stress, and the number of stress cycles leading to the destruction of the connecting part is determined empirically. The stress-number curve indicates the respective number of stress cycles leading to destruction as a function of the magnitude of the repeated stress. The lower the stress, the less the dependency of the stress period on the stress. Steel materials that have resisted about 2xc3x97106 stress cycles are also not destroyed after further cycles if the stress remains unchanged, so the fatigue strength can be assumed. For aluminum materials, the fatigue strength begins after about 107 stress cycles.
It is thus a relatively simple, fast process to determine the fatigue strength, that is, the maximum stress, of a connecting part without it being destroyed over a long period of time if the materials are not corroded. It is difficult, however, to ascertain the fatigue strength of corrodible connecting parts whose intended use subjects them to a certain corrosive environment. Corrosion can heavily influence fatigue strength. To this point, only certain corrosive environments were used to assess the effect of corrosion on the durability of a connecting part. One particular measure is the salt-spray fog test, in which a 3% NaCl solution is sprayed into the air while the connecting part is subjected to repeated mechanical stress.
A shortcoming of these tests, however, is that they do not allow for an extrapolation of the obtained values for a period of, for example, 30 years. In other words, it is not possible to determine the fatigue strength in a reasonable length of time. In addition, the tests are performed in standardized corrosive environments that often inadequately represent actual environments, so the obtained results are unusable.
DE 89 02 058 U1 discloses a method for determining the fatigue strength (compression-tension fatigue limit) of metallic materials that are exposed to repeated mechanical stresses in a certain corrosive environment.
DE-OS 16 73 274 describes a method for determining the fatigue strength of thick-walled semi-finished products that are exposed to a mechanical stress (tensile stress) in a certain corrosive environment.
Various connecting techniques are tested on a comparative basis through pre-corrosion and the recording of stress-number curves in E. Lachmann: Durability of bonded metal joints in motor vehicle construction, VIDE COUCHE MINCES, No. 272, Supp. S, August 1994-October 1994, pp. 277-283. A drawback of this investigation is that no dimension characteristic values can be determined, and the pre-corrosion period is very lengthy.
A similar method is described in Y. Oue et al.: Dependence of corrosion fatigue behaviour of friction-welded butt joints on friction welding procedure. Welding International, GB, Welding Institute, Abington, Vol. 10, No. 3, pp. 207-214. The pre-corrosion period is also unreasonably lengthy here.
A similar method is employed to test the resistance to vibration of base material and butt joints, as described in F.-J. Flossdorf et al.: Schwingfestigkeit wetterfester Baustxc3xa4hle nach langjxc3xa4hriger Bewitterung [Vibration Resistance of Weather-Proof Construction Steels after Long-Term Weathering]. Stahl und Eisen [Steel and Iron], DE, Verlag Stahleisen GmbH [publisher], Dxc3xcsseldorf, Vol. 117 (1997), No. 11, pp. 105-111. Lengthy exposure to weather effects the pre-corrosion.
It is therefore the object of the present invention to improve a method for determining the fatigue strength of a corrodible connecting part between at least two overlapping construction parts, the part being subjected to a certain corrosive environment during its intended use, with diagrams (stress-number curves) showing the magnitude of the repeated stress as a function of the number of load repetitions up to the point of destruction of the connecting part, such that the actual corrosion conditions are more carefully considered and an extrapolation of the measured values is possible, so it can be determined within a short period of time whether a certain connecting part can withstand a certain stress in a certain corrosive environment over a period of, for example, 30 years.
In accordance with the invention, this object generally is accomplished by a method of the type described initially above wherein: stress-number curves are determined for test pieces for different, stress-free pre-corrosion periods; from the stress-number curves, the time after which the pre-corrosion period no longer affects the course of a subsequently-recorded stress-number curve is determined; afterward, a stress-free pre-corrosion of the connecting part is performed for a particular duration in an environment that corresponds to the defined corrosive environment, after which the duration of the pre-corrosion no longer affects the course of a subsequently-recorded stress-number curve; and, in order to record a stress-number curve, the pre-corroded connecting part is subsequently subjected to a predetermined, repeated mechanical stress in an environment that corresponds to the specific corrosive environment until the fatigue strength is attained. Advantageous modifications of the method in accordance with the invention are described.
For a specified period of time, the connecting part is subjected to a stress-free pre-corrosion in an environment that corresponds to the specified corrosive environment, after which the duration of the pre-corrosion virtually no longer influences the course of a subsequently-recorded stress-number curve. For recording a stress-number curve, the pre-corroded connecting part is then subjected to a predetermined, repeated mechanical stress in an environment that corresponds to the certain corrosive environment until the fatigue strength is attained. This occurs before the actual stress test. Consequently, the connecting part is already in such a corroded state that additional corrosion does not worsen the stability of the connection, that is, prior to the start of the stress test, the degree of corrosion that is the least favorable for the strength has already been attained. Surprisingly, it has been seen that this state can be attained after a relatively short period of time. Furthermore, because a corrosive environment that corresponds to the corrosive environment to which the connecting part is exposed during its intended use is selected for the pre-corrosion, a corrosion that corresponds to the corrosion during actual operation can be simulated; thus, more reliable values are obtained. In the subsequent stress test, stress-number curves that correspond to the stress-number curves for non-corroded connecting parts are obtained, which permits a problem-free extrapolation of a fatigue stress.
The pre-corrosion is preferably effected at a temperature higher than room temperature. This accelerates the corrosion, or shortens the pre-corrosion time; for example, at a temperature of 60xc2x0 C., the pre-corrosion time can be reduced to about {fraction (1/10)} of the time at room temperature.